Distributed feedback (DFB) semiconductor lasers have been developed and are described, for example, in "GaInAsP/InP Phase-Adjusted Distributed Feedback Lasers With a Step-Like Nonuniform Stripe Width Structure" by Soda et al., Electronics Letters, 22nd November 1984, Vol. 20, pp. 1016-1018. In such DFB lasers, a longitudinally disposed diffraction lattice is associated with the laser active layer for Bragg reflecting light of the design wavelength, thereby to exhibit wavelength selectivity for the output laser oscillation. It has been found, however, that the longitudinal mode oscillation does not occur at precisely the Bragg wavelength (twice the length of a period of the diffraction lattice), but exhibits two peaks, one on either side of the Bragg wavelength and displaced from the Bragg wavelength by equal amounts. A possible reason for such behavior is that photon emissions at the Bragg wavelength which reciprocate in the resonator return to their original position shifted by .pi. radians and thus cancel. The result is that oscillation at the Bragg wavelength is cancelled, and oscillation then is at two peaks bracketing the Bragg wavelength separated by a null at the Bragg wavelength. It is appreciated that the two peak mode is undesirable and efforts have been made to stabilize the distributed feedback laser at a single longitudinal mode.
One such effort is illustrated in the aforementioned Soda et al. article. That article proposes introducing a phase shift in the laser stripe of .pi./2 radians resulting in single longitudinal mode operation at the Bragg is desirable, the means of achieving it introduces other undesirable characteristics. More particularly, in order to introduce a phase shift into the laser stripe, the stripe was made nonuniform in shape, having a step-like increase in stripe width for a portion of the stripe length. That increase in stripe width reduced the laser efficiency because current which flowed through the added step-like portions does not contribute to the laser output. The step also introduces the possibility of scattering light at the steps and thereby disturbing the radiation pattern.
The laser described in the aforementioned article utilized InGaAsP series material. Were the phase adjusting technique to be applied to a AlGaAs type laser, the structure would appear as illustrated in FIG. 2. The laser of FIG. 2 is based on an n-type GaAs substrate 1. Epitaxially ground on the substrate 1 is an n-type AlGaAs buffer layer 2. Corrugations 3 of the desired depth and period are then formed in the buffer layer 2 such as by using holographic photolithography to form a mask for the pattern and etching to form the pattern itself.
Subsequent to formation of the corrugation pattern 3, a second phase epitaxial growth process is utilized to successively deposit n-type AlGaAs guide layer 4, AlGaAs active layer 5, p-type AlGaAs cladding layer 6, and p-type GaAs contact layer 7. The molar proportion of aluminum in the active layer 5 is substantially less than that in the cladding layer 6 to aid in confining emitted photons within the active and guide layers 5, 4.
After the second phase epitaxial growth phase is completed, the device is masked and etched to remove longitudinal side portions 8, 9 leaving a stripe-like central mesa portion 10. It is seen from the drawing that formation of the mesa 10 is accomplished by removal of portions of layers 2, 4, 5, 6 and 7. In order to achieve the desired phase shift, the mesa 10 is not formed in the shape of a uniform stripe as is conventional, but has a step-like expanded portion 12 intermediate a pair of uniform stripe width regions 13. In the disclosed embodiment, the thickened portion 12 is about 60 microns in length out of a total stripe length of 400 microns, and is about 3 microns wide as compared to the 2 microns for the uniform stripe width portion.
After etching of the mesa, a third epitaxial growth phase is conducted to cover the two sides of the mesa, thereby burying the heterojunction formed in the active layer 5 between the p-type cladding layer 6 and the n-type guide layer 4. A first p-type AlGaAs embedded layer 15 is grown followed by an n-type AlGaAs embedded layer 16 having an upper surface which is substantially planar with the contact layer 7. The embedded layer 15 serves to stabilize the transverse mode, and the reversed bias p-n junction formed between the layers 15-16 reduces leakage current through the device.
Following completion of the final epitaxial growth phase, p side and n side electrodes 17, 18 are formed on the upper and lower surfaces, respectively, of the laser. Anti-reflective films 19 such as Si.sub.3 N.sub.4 are formed on the laser facets to eliminate the influences of reflectivities of the laser end surfaces.
In operation, when a d-c voltage source is connected to the electrodes 17, 18 to forward bias the p-n junction formed between the layers 6 and 4, carriers are injected into the active layer 5 causing the production of coherent radiation. Photons are generated and are confined within the stripe section of the active and guide layers 5, 4 oscillating between facets until they are ultimately ejected through one of the facets to produce a beam of coherent radiation. The period of the corrugations in the layer 2 determines the wavelength of the light which is produced. By virtue of the greater stripe width at the step-like portion 12, the propagation constant at that portion is different than that of the uniform stripe regions 13. Thus, the difference in propagation constant .DELTA..beta. multiplied by the effective length of the step-like portion 12 creates a phase shift for light traveling in the stripe region. When this phase shift is adjusted to be about .pi./2 radians, such as by making the thickened portion about 60 microns long as noted above, the phase shift approximates .pi./2 and it is found that the longitudinal mode of the laser is stabilized at a single frequency which is at about the Bragg wavelength.
However, it will also be appreciated that the only current which contributes to light output is that which flows through the uniform width section of the laser stripe, i.e., the portions 13 and the central portion of the region 12. Current, however, also flows through the step portions of the region 12, i.e., the portions which extend beyond the uniform portions, and that current does not contribute to laser output light. That portion of the current can be considered reactive current, i.e., current which is supplied to the laser but does not contribute to light output. Instead, that current is dissipated as heat, and that is undesirable because it causes an increase in operating temperature of the laser device. In a laser with a 2 micron by 400 uniform stripe size and a 3 micron by 60 micron steplike thickened portion, approximately 7.5% of the laser area which is passing current produces no light output. A further disadvantage also appears to be introduced by the thickened portion, namely, the discontinuity in width has an effect in scattering the radiation which is produced, thereby disturbing the pattern of light output.